This invention relates to a method of and apparatus for interpolating missing lines of a video signal obtained by line scanning such as a broadcast television signal.
Many television processes require the interpolation of television (TV) pictures, to create a signal like one which would have been generated if the picture had been scanned in a different way. For example digital video effects systems change the size and shape of TV pictures, and standards conversion changes the number of lines per field and/or the number of fields per second. There is also considerable potential benefit to the television viewer from display systems which increase the number of lines and/or fields in the display to reduce the visibility of line structure, flicker and twitter.
Spatial interpolation, that is, interpolation within a single field, would be relatively straightforward if it were not for the use of interlace in all current broadcast TV systems. Each field of an interlaced TV picture contains only half of the lines of a complete picture. This makes interpolation difficult because the lines of each field do not contain the full vertical resolution. The missing information is carried by the interlaced lines of the adjacent fields, but these may differ from the current field because of movement.
This specification is concerned with a method of interpolating the "missing" lines needed to convert interlaced pictures into sequentially scanned pictures. Once the missing lines have been added any further interpolation is straightforward. Any subsequent interpolation may in practice be combined with the interpolation described in this specification, but the two operations are considered separately for ease of description.
Characteristics of known methods are illustrated in FIGS. 1 and 2. In FIG. 1, the various diagrams are plots showing the vertical positions of lines on the Y-axis against time in terms of fields on the X-axis. In FIG. 1 each input line is shown by an X and each "missing" line to be generated as an output line by an O. The "current" output line is assumed to be the O with a dot in it. FIG. 2 shows the response to vertical detail of the different systems, with the response plotted on the Y-axis in terms of a percentage of perfect (100%) response, and vertical frequency plotted on the X-axis in terms of a percentage of the maximum definition of which the system is capable in the vertical direction, i.e., in the 625 line TV system used for broadcasting in the U.K., 100% is 312.5 cycles per picture height.
If the missing lines are interpolated from lines of the current field, as illustrated at (a) in FIG. 1 of the drawings, then the vertical resolution is limited, as shown at (a) in FIG. 2. If on the other hand the missing lines are interpolated from the adjacent fields, as illustrated at (b) in FIG. 1, then although the response at 0 Hz (stationary pictures) is perfect, as shown at (b) in FIG. 2, at all other temporal frequencies the response falls off, becoming zero at 25 Hz. These higher temporal frequency components are created by movement and their removal results in serious movement blur, which is roughly equivalent to doubling the integration time of the camera. Combinations of the two methods, as illustrated at (c) in FIG. 1, usually give a combination of impairments. This is shown at (c) and (d) in FIG. 2, which show the vertical frequency responses at 0 and 25 Hz respectively.
This has in the past led to the assumption that some form of adaption, to distinguish moving areas of the picture from stationary areas, is essential, so that the most appropriate form of interpolation can be used in each area. We have now appreciated that it is after all possible to devise a combined spatio-temporal interpolator which gives improved vertical resolution, without any subjectively-serious movement blur.